228 ' METALLURGY'OF IRON" AND STEEL.
additions of limestone were 23,240 pounds, giving 13,000 pounds of CaO, and if the slag contained 39.07 per cent, of CaO the weight of the slag would be 33,300 pounds. There were 482-7 pounds of silica added and the slag was supposed to contain 12.75 per cent, of Si02. This calls for 37,860 pounds of slag, so that the weight of the slag found by these two different- methods -agrees within 12 per cent. On a different series of twenty-seven heats . Mr. Talbot gives the weight of the slag, and if we calculate this so as to be in proportion to the 'weight of metal, the slag 'would weigh 42,000 pounds, when, by our two theoretical calculations founded on other heats it would be 33,300 and 37,860' pounds. Variations in the pig-iron might account for greater discrepancies than these.
We may say with some certainty that in the pig-and-ore process, with' molten pig-iron in a basic furnace, the oxidation of • the •metalloids is mainly due to the ore'^and very little: to the flame. When pig-iron is ^charged cold there is more oxidation during melting, and the amount of ore will, be reduced. .When a mixture of pig and scrap is charged, the time of melting is lengthened and the: stock is exposed'longer to the flame and the oxidation: done by the gases is greater.
SEC. Xllg.—Gain in weight-~by reduction of ore.—When iron ore is added to an open-hearth bath, the metalloids are oxidized and the iron is reduced. X certain amount of the oxide is lost in the slag, this amount varying with the amount and the nature of the slag. An open-hearth slag'will usually carry about a certain percentage of iron, and the greater the quantity of slag the greater the loss of iroii. '.'Pvery pound of silicon in the pig-iron produces silica and inc.r|a||s,the amount of lime necessary and increases the amount 'of iron-ihat must accompany the resultant cinder. Every pound of silica in the ore and in the lime, and every pound from the erosion of the bottom or the melting of the roof, increases the volume of the slag and the loss of iron. Given the weigHt of silica present, together with "the percentage of'silica in the slag, and the weight of the slag may be found by simple division. A simpler w'ay of making a rough estimate of the weight of a basic slag is to double the amount of burned lime used, or if limestone is added, the weight of the: slag" will be about 25 'per cent, more than the weight of the-stone, for limestone is a little over half CaO and burned lime is somewhat less 'than'half CaO, owing to incomplete burning and